Machines such as, for example, wheel loaders, dozers, backhoes, dump trucks, and other heavy equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a transmission to one or more ground engaging devices. Such engines typically produce a maximum power output at a single engine speed. Often, for increased engine efficiency, these machines utilize automatic type transmissions such as, for example, continuously variable transmissions (CVT) that provide an infinite number of output ratios within their ratio ranges.
An existing control strategy, typically employed by existing CVT control systems in response to a vehicular acceleration request, maintains an associated engine at the speed producing the maximum power output for that engine while increasing the transmission ratio. As the vehicle's speed increases, the engine speed is kept constant until the transmission ratio reaches a maximum. If the vehicle's maximum speed has not been reached at this point, the existing control strategy increases the engine speed.
Another CVT control strategy employed in response to a vehicular acceleration is disclosed in U.S. Pat. No. 6,066,070 (the '070 patent) issued to Ito et al. on May 23, 2000. The control system disclosed in the '070 patent references an acceleration map when receiving an acceleration request. Based on the map, the control system increases engine speed in proportion to the increase in vehicular speed. At the same time, the transmission ratio of the CVT is manipulated to generate a desired torque output. The control system continues the acceleration event until either the acceleration request has been terminated or the maximum engine speed and transmission ratio have been reached. In addition, the engine speed utilized at the end of the acceleration event is maintained until either another acceleration request is received or a deceleration request is received.
Although the control strategy disclosed in '070 patent and the other existing CVT control strategies may adequately operate a power system at lower vehicular speeds, such control strategies may produce sluggish performances at higher vehicular speeds. In particular, because the engine speed varies, it is no longer set to the speed that produces the maximum engine power. Therefore, the power output of the transmission is reduced. If the vehicle meets increased external resistance such as a hill, the transmission may not be able to maintain the maximum speed. In addition, as the vehicular speed increases, rolling resistance and internal friction experienced in the wheel and axle increase and may act against the transmission's power output.
The disclosed system is directed to overcoming one or more of the problems set forth above.